Ishai posted a very nice article about a print list option in SharePoint. Besides the fact that it is a useful option, I demonstrates the extensibility of the SharePoint UI using features very well.
"The solution will be deployed as a wss feature, which allows us an easy way to add a menu item to the sharepoint menus. The feature will define that the item we want to add will be added to the actions menu of all lists, in a site collection. You can ofcourse change it so that it behaves differently and connects only to lists of a certain type if you so wish, or maybe move the menu item to a different place. I recommend reviewing the msdn article on the possible configurations."
Sometime soon I will post some articles about the feature framework as well, with some interesting examples.